1. Field of the Invention
The present invention is directed to an upper feed device for a sewing machine and, more particularly, to an upper feed device for a sewing machine which feeds cloth by collaboration of an upper feed dog and a lower feed dog which make ellipsoidal motions in four directions consisting of longitudinal movements and vertical movements.
2. Description of the Prior Art
A known upper feed device for a sewing machine comprises a crank which swings longitudinally in cooperation with a main shaft, an upper feed arm which has a base portion which pivots rotatably on the crank in a vertical direction, an upper feed dog disposed at the free end of the upper feed arm and positioned opposite a lower feed dog, a spring device for pushing down the upper feed arm, and a crank which swings in the vertical direction in cooperation with the main shaft and moves the upper feed arm vertically while the upper feed arm is pushed down by the spring device. In U.S. Pat. No. 4 589 364, in the mechanism for moving the upper feed arm vertically, a clutch mechanism comprises an abutment on a collar affixed to a swinging shaft and an abutment on a lever rotatably disposed on the swinging shaft. Also, a roller which pivots on the crank pin of the crank and moves the upper feed arm vertically is fitted in the guide groove formed by the guide disposed below the upper feed arm.
In the above devices, during the descent of the upper feed dog to the bottom end point, where it engages with the cloth on the throat plate and further descent is prevented, the crank pin separates from the upper feed arm in the former type, or the abutment on the lever separates from the abutment on the collar in the latter type with the clutch mechanism, and the crank pin, separated from the upper feed arm, reaches the bottom end point and pivots reversibly and engages with and lifts the upper feed arm, or the abutment of the lever separates from the abutment of the collar and engages and lifts the upper feed arm, so that the upper feed dog rises above the cloth on the throat plate. In this manner, that is, while the upper feed dog is engaged with the cloth on the throat plate, cloth feeding is carried out.
FIG. 7 shows the track of the upper feed dog, in which the cloth is fed between point A, where the upper feed dog descends and engages with the cloth on the throat plate, and point B where it has ascended from the cloth on the throat plate. The horizontal line segment AB indicates the cloth feed amount. In FIG. 7, when it is attempted to feed the cloth at the position of line segment CD by lowering the position of the upper feed dog to engage with the cloth, the feed efficiency, that is, the cloth feed amount increases, but the cloth's engagement with the ascending lower feed dog becomes stronger, and the impulsive cloth gripping force is intensified and, as a result, the cloth is damaged by the biting of the upper and lower feed dogs.
To solve this problem, Japanese Laid-open Utility Model No. 63-55972 proposes the provision of a cam groove in a special shape and a leaf spring for the guide groove to engage with the crank pin in order to reduce the impulsive force, but the fabrication and processing of the cam groove are rather difficult.